#1 The better your materials, the better the result.

If you want to try and make a vorpal sword from simple iron and a stick you are going to fail, miserably. The more rare/special/freely given your materials, the better the outcome. Also the more skill that has been put into them matters too.

#2 The roll of the die matters only a little.

You can ask "If I try making X with A/B/C materials how good is my chance?" and I'll give you a general idea of how good your odds are, and possibly some idea of why. The chance will end up being a percentage based on all the accumulated materials and workmanship. (Yes, it will be very possibly to have a 100% chance or better to craft something if you have rare materials and a high level and you want a simple item.)

#3 Keep a list of what you have gained, how it has been worked and what you want to use it for.

IT IS YOUR RESPONSIBILITY TO KEEP TRACK OF THIS! I won't remember every bit of spider venom and where it came from.

#4 The DM reserves the right to twist magic however feels right.

If you keep the spider silk soaked in the blink dog blood you might find strange things happen in the end. Maybe good, maybe bad.

#5 Some things will be specific, some things will be general.

If you go foraging for plants I might say, "You find enough misc. herbs to make one basic vial of abjuration oil. And 2 Crimbol Mushrooms that have strong Divination properties" The more specific the ingredient the more valuable or useful. We can get as detailed with this as you want.

#6 This is incomplete

I am intentionally leaving these rules very open to modification. But my goal is to make magical item creation more of a gathering quest than just a money sink. However you can always buy ingredients.